01 - 
                Toss salt, baking soda, cocoa powder, and flour in a bowl, then mix with a whisk until it all looks the same.
              
              
              
                02 - 
                Grab your mixer and whip the butter with sugar until you get a light, fluffy look—usually takes about 3 or 4 minutes.
              
              
              
                03 - 
                Crack in the eggs one after another, mixing real good each time. Pour in the vanilla until you can't see it anymore.
              
              
              
                04 - 
                Slowly dump the dry stuff into the wet, and run your mixer on low, just until it comes together. Don't go crazy mixing or cookies get tough.
              
              
              
                05 - 
                Gently fold in marshmallow fluff so you've got cool swirls, or save it for after chilling if you'd rather.
              
              
              
                06 - 
                Wrap the dough up and stick it in the fridge for an hour. This helps so it won't spread too much when you bake it.
              
              
              
                07 - 
                Turn your oven on to 175°C. Lay out parchment or silicone mats on two trays so cookies don't stick.
              
              
              
                08 - 
                Scoop out dough balls and roll them. Put on trays spaced apart by about 5 cm. Push in the middle with your thumb, spoon a little marshmallow inside, swirl with a toothpick if you want.
              
              
              
                09 - 
                Pop the trays in the oven for 10–12 minutes. Edges should look done but middles still kinda soft. For super gooey cookies, add a bit more marshmallow fluff during baking.
              
              
              
                10 - 
                Let cookies chill on the pan about 5 minutes. Move to a cooling rack and leave there to finish cooling and get chewy.